When boreholes are drilled into subterranean reservoirs, e.g. hydrocarbon reservoirs, the drill bit during drilling is conventionally surrounded by a drilling fluid which is continuously pumped down to the distal end of the borehole and back to the drilling head, e.g. down through the hollow drill string which carries the drill bit and back through the annulus between the drill string and the inner wall of the borehole. The circuit from drill head down hole and back to the drill head may take the drilling fluid several hours to complete.
One of the functions of the drilling fluid is to carry the debris and cuttings created by drilling out of the borehole and accordingly, on reaching the drill head, the drilling fluid is generally screened to remove debris before being returned to a holding tank, generally referred to as a drilling fluid reservoir, from which it can be recycled down hole. It is important that debris and cuttings are removed efficiently from the wellbore because they can interfere with the operation of the drill bit and can significantly impede the progress of the drilling operation.
In terms of other functions, the drilling fluid pumped into a wellbore also helps to drive the drill bit into the wellbore and to cool and lubricate the drill bit. Further, it may be applied to counterbalance hydrostatic pressure in the wellbore thereby preventing blow out. The drilling fluid also functions to maintain borehole stability by generating a pressure against the wellbore wall and thereby prevent it from collapsing. It also provides fluid loss control, i.e. it prevents loss of fluid into the formation, and it provides chemical stability to the formation thereby preventing chemically induced instability of the wellbore.
These functions should ideally be achieved whilst minimising formation damage and thus the subsequent impairment of the production from a well or the ability to inject fluids such as gas or water for production support into the well. Damage may be caused by solid particles contained in the drilling fluids or drilling fluids filtrate that enter the formation. These drilling fluids components may trigger reactions such as plugging of flow paths through said solid particles, the mobilization of fine particles, the swelling of clay minerals, changes in fluids saturations, the generation of stable emulsion droplets, and the precipitation of organic or inorganic scale. Each of these reactions has the potential to reduce the effective permeability of the formations that are entered by the wellbore either for production of the formation contents or injection of gas or water.
The particular composition of the drilling fluid can impact significantly on its ability to perform these various functions whilst minimising formation damage. At the same time, downhole conditions such as wellbore mineralogy, temperature and pressure, drilling rates and trajectory, well length and volume etc, can affect fluid effectiveness. It is clearly desirable to use a drilling fluid that is suitable for given downhole conditions and achieves one and ideally all of the functions above.
Drilling fluids are typically water or oil based compositions comprising a mixture of chemicals designed to achieve the above-described range of functions. Drilling fluids are discussed for example in Darley and Gray “Composition and Properties of Drilling and Completion Fluids”, Gulf Professional Publishing, 5th ed., 1988. Fluids may be formed, for example, with certain viscosities, densities, fluid loss control properties and chemical contents in order to try to provide the desired performance. However, well and wellbore conditions continuously change during the performance of a wellbore operation as, for example, drilling progresses and different geological intervals are entered. Cuttings and debris from the formation may also become mixed into and suspended in the fluid and re-circulated back into the borehole if they are not effectively removed at the surface.
Drilling fluids are complex chemical mixtures designed to achieve a variety of tasks and in use their performance can worsen as their physiochemical properties alter. Drilling fluids in motion experience mechanical wear through the drilling and pumping process that causes the degradation or deterioration of drilling fluid components. Also, the interaction between the drilling fluid and subterranean formation causes the removal or degradation of drilling fluid components by the formation due to reactions between formation and fluid. As components disappear, degrade or deteriorate, they cannot efficiently maintain the physiochemical properties of the fluid and have to be replaced by new components. Wellbore pressure and temperature impact on the fluid as well as the nature of the formation. Accordingly the properties, e.g. viscosity, of the drilling fluid may change significantly during the drilling operation affecting its subsequent performance when it is recirculated back into a wellbore.
As a result, it can be difficult for operators to select an appropriate drilling fluid for an operation and once a particular fluid is chosen by an operator, it is uncertain whether it is going to continue to be an appropriate drilling fluid once subjected to the wellbore environment. As a result, the productivity of the drilling operation can be detrimentally affected. It is thus conventional for samples of returning drilling fluid to be taken and subjected to a battery of tests to determine values for its properties. Based on the results from those tests, the drilling fluid may be treated, e.g. by the addition of various components, so as to bring the values of these physiochemical properties back into the appropriate ranges for recycling down hole.
Examples of the physiochemical properties that are currently measured include: mud weight, viscosity, gel strength, water content, oil content, oil/water ratio, solids content, sand content, barite content, pH, methylene blue capacity, filtrate alkalinity, mud alkalinity, salt content, chloride content, potassium content, lime content, barite sag stability, etc. Some of these properties are time-invariant; however others are kinetic in that a measurable property develops over time in a static or agitated sample. Thus for example drilling fluid at rest develops a gel-like consistency which is broken when the fluid is agitated. Similarly, aged drilling fluid at rest has a tendency to “sag”; the high density solids such as barite, added to increase the pressure at the drilling site, develop an undesirable tendency to settle out.
This subjection to a battery of tests and the subsequent manual adjustment of the drilling fluid is, however, time consuming and labour intensive and does not allow for rapid intervention if a sudden change in properties occurs.
We have found that this problem may be addressed by monitoring physiochemical properties of the drilling fluid using nuclear magnetic resonance (NMR), in particular low field NMR. Advantageously this enables the physiochemical properties of the drilling fluid to be monitored during the drilling phase and if necessary for rapid intervention to be carried out to ensure that the properties of the drilling fluid are optimised.
NMR has previously been used in down-hole monitoring of liquids entering the borehole from the surrounding matrix, i.e. during a production or completion phase. This is, for example, disclosed in US2008/0035332 wherein NMR is one of the methods used to take measurements on reservoir fluids pumped into the flowline of a fluid sampling tool. The use of NMR has not, however, previously been disclosed for on-line monitoring of out-of-hole drilling fluids during a drilling phase.